Do fish enhance tank mixing? (original) (raw)

The design of fish rearing tanks represents a critical stage in the development of optimal aquaculture systems, especially in the context of recirculating systems. Poor hydrodynamics can compromise water quality, waste management and the physiology and behaviour of fish, and thence, production potential and operational profitability. The hydrodynamic performance of tanks, therefore, represents an important parameter during the tank design process. Because there are significant complexities in combining the rigid principles of hydrodynamics with the stochastic behaviour of fish, however, most data upon tank hydrokinetics has been derived using tanks void of fish. Clearly, the presence of randomly moving objects, such as fish, in a water column will influence not only tank volumes by displacing water, but due to their activity, water dynamics and associated in-tank processes. In order to determine the impact of fish presence upon tank hydrodynamics, Rhodamine fluorometry was employed to examine mixing within a recirculating aquaculture system. Two different methods were compared, traditional, outlet-based measurements and a technique that employed in-tank data acquisition. Circular tanks were employed during data collection either in the presence or absence of experimental fish-red drum Sciaenops ocellatus (n =36; 5 kg total wet wt); and at two flow rates. Irrespective of flow rate, the presence of fish dramatically enhanced the mixing process (P b 0.001), with mixing times in tanks with fish being one-third that for tanks without animals. In-tank dispersion coefficients and dispersion numbers also differed (P b 0.001) in the presence of fish, irrespective of flow. Presence or absence of fish had no effect upon hydraulic residence or circulation times. Unlike measurements at the outlet, in-tank observations were more able to isolate the effects of stochastic, fish-induced mixing, from deterministic, hydrodynamic mixing.